The present invention pertains to radio signal receivers and more particularly a global positioning system (GPS) digital receiver.
The global positioning system is a constellation of satellites which transmits navigation information via radio signals. Time and position may be calculated by receivers which are able to receive and process these radio signals. The satellites of the GPS constellation broadcast two BPSK modulated signals at L-band, 1575.42 megahertz (L1) and 1227.6 megahertz (L2). The modulated signals include psuedorandom noise codes and data. The L1 signal carrier is modulated in quadrature with both a clear acquisition code (CA code) and a precise code (P code). The chipping rate of the CA code is 1.023 megahertz and the P code is 10.23 megahertz. The L2 frequency is modulated with only one code, normally the P code.
It is necessary to track at least four satellites of the GPS constellation in order to compute a GPS receiver's position in three dimensions and determine time. One method of tracking at least four satellites, is to assign one receiver channel to track each of the satellites. This is termed continuous tracking. Another method of tracking four satellites with fewer than four channels requires time-sharing of the channels among the four or more satellite signals. This type of tracking is referred to as sequencing.
Data loss may result from the time required to lock-up or synchronize to a particular satellite signal during the sequencing process. Accordingly, such operation requires circuitry which may be set up rapidly by the control circuitry when the time-sharing operation causes switching from one satellite to another. In addition for digital receivers, high Doppler frequencies require high-speed logic to rapidly process the digitized signals.
Further, the time required to re-acquire the data transmission of a satellite including compensation for Doppler shifting is related to the accuracy with which a GPS receiver may determine its precise location. Sequencing through a number of satellite transmissions requires the sequence to quickly lock-up to the satellite's transmission to obtain the satellite's Doppler shift, pseudorange data and the broadcast data.
Accordingly, it is an advantage of the present invention to provide a sequenced GPS receiver which rapidly re-acquires each satellite's data transmission sequentially to accurately perform self position location.